Gas-sensitive field-effect transistors based on semiconductors are increasingly being used in sensor systems. The application of a test species that is to be detected, in this context, for instance, a gas or a liquid, or a gas/liquid mixture, usually leads to a change in the channel impedance, and thus to a change in the current flowing from the source electrode to the drain electrode through the transistor. Such a field-effect transistor is known, for instance, from U.S. Pat. No. 5,698,771. The use of field-effect transistors is possible at up to 800° C. in sensor system applications, if semiconductor substances having a band gap of more than 2 eV, such as gallium nitride or silicon carbide, are used.
At the working point selected, the channel current of the field-effect transistor is frequently greater by some orders of magnitude (usually 103) than the change in the channel current because of the application of the test species. From this there comes about a great requirement for current measurement. In addition, a problem arises that the offset is able to be influenced by external interferences, so-called noise. Such external interference influences are, for instance, temperature changes or sensor degradation, which lead to changes in the channel current, and are not based on the presence of test species. Based on the given signal-offset ratio, the change in the channel current by interference influences may be of the same order of magnitude, or even greater in the least favorable case, than the change that takes place owing to the presence of test species. Since one cannot exclude these interference influences completely, the error in the measuring signal connected therewith may be large, and may prevent a usable measurement of the test species, in the least favorable case.
In order to compensate for interference influences and offsets, it is possible, for example, to use a field-effect transistor acting as a reference element, which is insensitive with respect to the substances to be detected. The reference element is preferably identical to the field-effect transistor acting as the measuring sensor with regard to its semiconductor patterns, geometric dimensions and electrical characteristics. Both field-effect transistors have the same zero signal, because of the same electrical characteristics. When the two field-effect transistors are slightly separated spatially, there also exists good heat coupling. This is a given, for instance, in response to the integration of the components on a chip. Because of this, the two field-effect transistors experience the same interference influences. A difference in the channel current of the field-effect transistor acting as measuring sensor may then be attributed only to the presence of the substances that are to be detected.
The passivating of field-effect transistors so as to make them into reference elements is accomplished according to the related art in a semiconductor process, with the aid of dielectric layers. These are generally deposited using thin-film techniques. However, such a passivating layer may, under certain circumstances, influence the electrical characteristics of the field-effect transistor. Thus, for example, stresses at the boundary layer between the passivating layer and the layer below it, in the case of piezoelectric semiconductor substances, such as gallium nitride, may lead to a change in the field-effect transistor channel. In addition, dielectric passivating layers frequently have electron states which are able to store loads, and are therefore able to influence the electric field under the gate electrode.
In addition, the passivating of a field-effect transistor used as a reference element on an integrated chip is very costly. Thus, process technology restrictions do not permit complete lateral patterning of the passivation, for example, and, on the other hand, for example, process parameters, such as high temperatures during the depositing of the passivation, damage the chemically sensitive gate of the measuring sensor. For this reason, field-effect transistors acting as a reference element and field-effect transistors acting as a measuring sensor have to be processed separately from one another. Under certain circumstances, this may lead to the field-effect transistors no longer being identical, possibly having different electrical characteristics.